A laminated glass is safe because even it is damaged from an external impact, few fragments thereof are scattered. It, therefore, has conventionally been employed widely as window for vehicles such as automobiles, aircraft, buildings and the like. Example of such laminated glass includes those obtained by interposing an interlayer film for a laminated glass, which hereinafter is sometimes referred to as an “interlayer film”, made of polyvinyl acetal resin, for example, polyvinyl butyral resin plasticized with a plasticizer, between at least a pair of glass sheets and then uniting them.
The laminated glass using therein such interlayer film is superior in safety, but it has a problem of poor heat insulation. Generally, among lights, an infrared radiation having a wavelength of 780 nm or more demonstrates, in comparison to an ultraviolet radiation, a greater thermal action even though it has an energy as small as about 10% of that of ultraviolet radiations, and once it is absorbed into a material, it is released as heat to cause an temperature increase. The infrared radiation is therefore called a heat ray. Accordingly, if a front glass or a side glass of an automobile is made possible to shield the infrared radiation shone thereto, the heat shield property is improved and the temperature increase inside of the automobile can be inhibited.
As a glass sheet with improved heat insulation-property, a heat ray-cutting glass and the like is available commercially. The heat ray-cutting glass is a product obtained by applying a multilayer coating of metal/metal oxide onto a surface of a glass sheet by metallic coating, sputtering or the like for the purpose of shielding direct solar radiation. Since a coating layer is less resistant to external scratches and also to chemicals, the heat ray-cutting glass is fabricated into a laminated glass by laminating an interlayer film such as a plasticized polyvinyl butyral resin film. However, in practice, radiations within the mid-infrared region, in which region human beings feel the heat feeling most, are not cut effectively. In particular, radiations in regions where human beings feel the heat feeling through the temperature rise of the epidermis of the skin (1400 to 1600 nm and 1800 to 2000 nm) and radiations in regions where the radiations reach nerve endings in the deep layer of the skin and human beings feel a feeling of stimulation therefrom (1200 to 1400 nm, 1600 to 1800 nm and 2000 to 2400 nm) have not been fully cut.
Moreover, the heat ray-cutting glass with a multilayer coating of metal/metal oxide is problematic in that the glass is expensive; that the transparency (the transmittance of visible light) is low because the coating layer is thick; that the adhesion between the coating layer and the interlayer film is reduced to cause delamination or whitening of the interlayer film; and that penetration of electromagnetic waves, in particular, those within a communication wavelength region is inhibited and trouble will be caused on communication function of cellular phone, car navigation system, garage opener and automatic cash receiver and the like.
As solutions to such problems, Japanese Kokoku Publication Sho-61-52093, Japanese Kokai Publication Sho-64-36442 and the like propose laminated glasses using an interlayer film comprising a metallic coating polyester film interposed between plasticized polyvinylacetal resin sheets. These laminated glasses, however, are problematic in adhesion of the plasticized polyvinylacetal resin sheet to the polyester film and, therefore, cause delamination at their interfaces. Moreover, they are insufficient in electromagnetic wave permeabilities, in particular, permeabilities of electromagnetic waves within a communication wavelength region.
With advancement of the highly information-oriented society, increase in speed and improvement in performance have recently been required in the fields of information processing and information communication. In the field of information communication, the frequency used is shifting from the ultra-high frequency band (300 MHz to 1 GHz) to the quasi-microwave band (1 to 3 GHz) with increase in communication capacity of mobile communication equipment such as cellular phone and car navigation system. In late years, introduction of the ETC (Electric Toll Collection System), which has been put into practical use in Europe, is pushed forward also in Asian countries. The ETC is a system, which makes it possible to pay fees automatically and go through tollgates without stopping there by road-to-vehicle communicating between an antenna mounted at the gate of a tollbooth and on-board equipment mounted to a vehicle.
In Japan, that system is under trial use at 54 tollbooths in a metropolitan area in and around Chiba area since Apr. 24, 2000. The system is scheduled to be adopted at about 600 tollbooths on the Tomei, Meishin and Chuo Expressways in the fiscal 2001 and is also planned to be rolled out nationally so as to be adopted at 900 tollbooths by the end of March, 2003. The system under global standardization is the 5.8 GHz-band active system. Therefore, the permeability of the electromagnetic waves of this wavelength band, in particular, the permeability of the electromagnetic waves of a communication wave region will become very important. Such high frequency waves have a nature of being lost through their conversion into heat. Therefore, an efficient transmission of electric signals requires materials of small transmission loss. Low dielectric materials are demanded.
In addition to the above-mentioned, frequency band employed are a 2.5 GHz for VICS (vehicle information and communication system), a 3.5 MHz band and a 7 MHz band for amateur radio and a 10 MHz or less for an emergency communication frequency, respectively. Moreover, a 12 GHz band is used for satellite broadcasting.
Generally it is known that the dielectric loss is represented by the following formula (1):Dielectric loss=(27.3×f/C)×εγ1/2×tan δ  (1)
In formula (1), f, C, εγ and tan δ denote a frequency, an electrostatic capacity, a relative permittivity, and a dielectric dissipation factor, respectively.
According to formula (1), the dielectric loss becomes greater with increase of the frequency. When the dielectric loss increases, the action of absorbing high-frequency signals and converting into heat is enhanced and it becomes impossible to transmit signals efficiently. To keep the dielectric loss small, it is necessary to make the relative permittivity and the dielectric dissipation factor small. Since the dielectric loss is in direct proportion to the dielectric dissipation factor, whereas it is in proportion to the square root of the relative permittivity. Therefore, it is necessary to select a material of small dielectric dissipation factor for high frequency. The measurement of a dielectric constant, although being an indirect approach, will make it possible to evaluate the electromagnetic wave shielding performance. The relative permittivity serves mainly as an index of the reflectance of electromagnetic waves, whereas the dielectric dissipation factor serves mainly as an index of the absorptivity of electromagnetic waves.
However, in the heat ray-cutting glass in which a multilayer coating of metal/metal oxide is applied to a surface of a glass sheet and a laminated glass comprising a metallic coating polyester film interposed between plasticized polyvinylacetal resin sheets of the prior art described above, a heat ray-cutting material is metal and/or conductive metal oxide tin film. They, therefore, shield electromagnetic waves as well as heat rays and accordingly cannot satisfy both shielding of heat rays and permeability of electromagnetic waves (mainly of a communication wavelength region), namely, both a low relative permittivity and a low dielectric dissipation factor.
Moreover, conventionally used heat reflecting glass or heat reflecting laminated glass using a heat reflecting polyethylene terephthalate (PET) are problematic in processability, workability, productivity and the like and are further problematic in that they will cause troubles on communication functions such as cellular phone, car navigator, garage opener and electric toll collection system.
On the other hand, Japanese Kokai Publication 2001-302289 discloses a laminated glass in which metal oxide having a heat shield property such as tin-doped indium oxide is dispersed in its interlayer film. This laminated glass is superior in the heat shield property and the electromagnetic wave permeability because of the use of a heat shield interlayer film. However, in its durability tests to heat, light and the like, the durability test time and the deterioration in visible light transmittance are in proportion to each other. The laminated glass demonstrates a greater reduction in visible light transmittance in comparison to laminated glasses using normal interlayer films and it tends to greatly increase in yellow index value, which is an index of yellowish hue, and in b* value in the CIE1976 L*a*b* color system. For example, there is a legal provision on lower limit of visible light transmittance for use of laminated glass as a front glass of an automobile. Therefore, it is particularly important that the visible light transmittance does not change during a durability test. It is undesirable from the viewpoint of external appearance that a heat shield interlayer film, which is normally light blue, be yellowish or cause yellowing due to its weathering deterioration. However, the conventional interlayer film containing tin-doped indium oxide is problematic in that the durabilities of its optical qualities such as visible light transmittance, yellow index value and b* value in the CIE1976 L*a*b* color system.